1. Field of the Invention
The present invention relates to selected chemically modified hydroxy styrene polymer resins and their use in photoactive resist compositions.
2. Brief Description of Prior Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally, in the processes, a thin coating or film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate. In some instances, it may be desirable to bake the imaged coated substrate after the latent imaging step and before the developing step. This bake step is commonly called a post-exposure bake and is used to increase resolution.
There are two types of photoresist compositions----negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g., a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the nonexposed areas of the resist coating and the creation of a negative image in the photoresist coating, and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition is deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g., a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. This etchant solution or plasma gases etch the portion of the substrate where the photoresist coating is removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the etching step to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Positive-working photoresists are generally prepared by blending a suitable alkali-soluble binder resin (e.g., a phenol-formaldehyde novolak resin) with a photoactive compound (PAC) which converts from being insoluble to soluble in an alkaline aqueous developer solution after exposure to a light or energy source. The most common class of PAC's employed today for positive-working resists are quinone diazide esters of a polyhydroxy compound. Typical novolak resins used today for positive-working resins are made from various mixtures of ortho-cresol, meta-cresol, and para-cresol which are condensed with an aldehyde source (e.g., formaldehyde). The combination of xylenols with such ortho-, meta-, and para-cresol mixtures are also used in making novolak resins.
Novolaks made from mixtures of meta-cresol and para-cresol are particularly popular as binder resins for positive photoresists. They provide good lithographic performance with quinone diazide ester sensitizers. However, one deficiency of such novolak resins is their relatively wide molecular weight distribution.
In comparison, other types of alkaline-soluble polymers have also been looked at as binder resins for photoresists. For example, hydroxy styrene polymers, hydrolyzed styrene-maleic anhydride copolymers, styrene-maleimide copolymers and other polymers of styrene derivatives have been tested for lithographic performance.
None of these polymers are useful alone in commercial photoresists because of their extremely high solubility in alkali developers used in the lithographic process. However, they do have the advantage of having a narrower molecular weight distribution compared to novolaks which results in a sharper switching of dissolution of the polymers in alkali developers.
In the past, poly(hydroxy styrene) has been modified in various ways [see Information Bulletin of Poly(p-hydroxy styrene) published by R. W. Rupp and B. N. Shah, Celanese Advanced Technology]. In particular to reduce the alkali dissolution rate of poly(hydroxy styrene) polymers, the research staff at Hoechst AG has examined alkylation of such polymers particularly at the ortho position to the hydroxy group on the phenolic ring (EP 307-752, 1989; Proc. SPIE Vol. 1262, Advances in Resist Technology and Processing VII, 391-400, 1990). However, the resulting polymers still have a poor property of dissolution inhibition with quinone diazide ester photoactive sensitizers compared with conventional novolak polymers, according to Hitachi research group (see Proc. 4th Micro Process Conference held at Kanazawa, Japan, Jul. 15-18, 1991; A-7-3).
The present invention provides a way of obtaining the advantages of the cresol novolak chemistry along with the alkali-solubility characteristics of poly (hydroxy styrene) polymers to obtain a polymer resin having a predetermined narrow molecular weight distribution. These binder resins of the present invention have (1) good dissolution inhibition of binder resin with quinone diazide ester sensitizers, (2) good discrimination in dissolution rate between exposed and unexposed areas, (3) substantially perpendicular walls of photo image, and (4) no scum.